FIG. 9 shows an example of a conventional art backlight system illuminating a display device (not shown) (see Patent Reference 1). The backlight system has a light source 5 including one or a plurality of light emitting diodes (hereinafter, abbreviated as LED or LEDs) and a light guide plate 1 having an incident plane 3 receiving light emitted from the light source 5. A reflection sheet 7 is arranged facing a first plane 2 of the light guide plate 1. A diffusion sheet 8 is arranged facing a second plane 4 of the light guide plate 1. Two prism sheets 9 and 10 are arranged above the diffusion sheet. The second plane 4 is a plane facing the display device. The reference numeral 6 denotes the state of progression of a few of the light beams in the light guide plate 1.
As shown in FIG. 10, in the conventional art backlight system having the above-described structure, when viewed from the second plane 4, in other words, the display device, the illuminating light illuminating the display device is nonuniform. As shown in FIG. 10, in more detail, in the plane of the light guide plate 1 (the diffusion sheet 8, and the prism sheets 9 and 10 are removed), a hatched portion 20 is a region appearing bright viewed from the front (the upper side) and an unhatched brightness shortage portion 21 is caused in the portion near the incident plane 3. The backlight user identifies that the brightness of the illuminated plane is nonuniform and there is illumination nonuniformity. To reduce the illumination nonuniformity, the number of light sources 5 is increased so as to bring the adjacent light sources 5 closer to each other in order to reduce the proportion of the light beam 6 incident into the incident plane 3 at a large angle.
The reason why illumination nonuniformity is caused will be described based on the structure of the conventional art light guide plate shown in FIGS. 11A to 11C and FIG. 12. To efficiently emit an incident light flux from the second plane 4 (the lower side viewed in FIG. 11C), the conventional art light guide plate 1 has a plurality of prisms provided on the first plane 2. As shown in FIGS. 11A and 11C, the respective prisms are linearly extended over the overall width of the light guide plate 1 in parallel with the incident plane 3. As shown in FIG. 12, each of the prisms is a prism with a triangle cross section shape having a reflection plane 2a gently and downwardly tilted from the front position near the light incident plane 3 toward the rear position far from the incident plane 3, a back plane 2b steeply tilted from the rear position rearward of the light guide plate 1, and a ridge line 2c formed by the reflection plane 2a and the back plane 2b. The reflection plane 2a is formed at a tilt angle (mold tilt angle) α0 with respect to the second plane 4 of the light guide plate 1.
When the respective prisms are formed in parallel with the incident plane 3 of the light guide plate 1, as shown in FIG. 11A, a tilt angle (a substantial tilt angle α) on the reflection plane 2a with respect to horizontal light incident horizontally from the incident plane 3 is different depending on an incident angle θ of the light beam 6 (one light beam included in the light flux incident into the light guide plate 1) formed on the plane of the light guide plate 1. This will be described with reference to FIG. 13. When the mold tilt angle α0 on the reflection plane 2a of the prism formed on the light guide plate 1 is expressed by d/p, the substantial tilt angle α on the reflection plane 2a of the light beam 6 incident from the incident plane 3 into the light guide plate 1 at the incident angle θ is expressed by d/p1. Since p<p1, the substantial tilt angle α is smaller than the mold tilt angle α0. As shown in FIG. 11A, the substantial tilt angle α on the reflection plane 2a with respect to the light beam incident substantially parallel with a center line D (which is typically substantially matched with the center axis of the light flux of the light source 5) of the light guide plate 1 orthogonal to the incident plane 3 is very close to the mold tilt angle α0 of the prism. The substantial tilt angle α on the reflection plane 2a with respect to the light beam 6 incident at the incident angle θ is approximately, α=α0·cos θ. As the light incident angle θ increases, the substantial tilt angle α gradually decreases and the amount of light reflected on the reflection plane 2a is gradually reduced, so that the amount of light emitted from the second plane 4 is reduced.
FIG. 14 is a graph (curve F) showing the substantial tilt angle α on the reflection plane 2a with respect to the incident angle θ of the light beam in the conventional art prism. The substantial tilt angle α is largest when the incident angle θ=0 (the light beam 6 is incident into the incident plane 3 of the light guide plate almost at a right angle) (in this example, α0=approximately 2°), and gradually decreases as the incident angle θ of the light beam increases.
In the conventional art light guide plate 1 shown in FIGS. 11A to 11C and FIG. 12, the amount of light reflected on the reflection plane 2a for the light beam 6 incident into the incident plane 3 at the large incident angle θ is greatly reduced, which is a major cause of illumination nonuniformity. To make considerable compensation for illumination nonuniformity, a plurality of light sources 5 (LEDs) need to be arranged close to each other so as to prevent the incident angle θ of the light beam 6 from becoming large.    Patent Reference 1: Japanese Patent Application Laid-Open No. 2004-327096 (FIG. 17)